$(function(){     
  $('#btn11').click(function(){
  if($('#u1').height()==0){
      $('.u2').css("position","relative")    
  }else{
$('.u2').css("position","absolute")
} 
 })
 $('#u1').children('li').mouseenter(function(){
   $(this).children('.u2').css("display","block")
   $(this).children('.a1').css({"background-color":"#000","color":"rgb(255, 81, 0)"})
 }).mouseleave(function(){
  $(this).children('.u2').css("display","none")
  $(this).children('.a1').css({"background-color":"rgb(46, 46, 46)","color":"#fff"});
  $('#a3').css({"background-color":"#000","color":"rgb(255, 81, 0)"})
 })
  })
